Luis F. Artigas

The molecular phylogeny of the sand-dwelling dinoflagellate Planodinium striatum was investigated from isolates collected in the NE English Channel. In the SSU rRNA gene phylogeny, the sequences of Planodinium clustered with the type species of Plagiodinium, P. belizeanum, and more distantly related to Plagiodinium ballux. Although the SSU rRNA gene sequences of Planodinium showed a high percentage of identity (96%) with partial sequences of species of Podolampas (~1200 base pairs), the SSU rRNA gene phylogenies did not show a relationship with the clade of the Podolampadaceae (Podolampas, Blepharocysta, Roscoffia), neither with the sequences of the sanddwelling genus Cabra. The SSU rRNA gene sequences of Plagiodinium belizeanum and P. ballux showed a low percentage of identity (96%) clustering in separate clades in the SSU rRNA gene phylogeny with Planodinium. The morphology of P. ballux and its generic type differed in the plate arrangement and the morphology of the cingulum, amongst other features. Based on the morphological and molecular differences, we propose the new genus Chrysodinium gen. nov. for P. ballux, with the re-interpreted thecal plate formula Po 3’ 1’’ 7c 2s 5’’’ 1’’’’.

The North Sea and the English Channel are regions with a long tradition of plankton studies, where the colony-forming haptophyte Phaeocystis globosa dominates the spring phytoplankton blooms. Among its predators, we investigated an abundant unarmored dinoflagellate (~3000 cells per liter) in the North Sea in May 2019. It has been reported in the literature as Gymnodinium heterostriatum or G. striatissimum, and often identified as Gyrodinium spirale. Phylogenetic analyses using the small-, large subunit- and Internal Transcriber Spacers of the ribosomal RNA (SSU-, LSU-, ITS rRNA) gene sequences indicate that our isolates clustered within the Gyrodinium clade. The new sequences formed a sister group with sequences of the freshwater taxon Gyrodinium helveticum, being one of the infrequent marine-freshwater transitions in the microbial world. This isolate is the first characterized member of a clade of numerous environmental sequences widely distributed from cold to tropical seas. This common and abundant taxon has received several names due to its morphological plasticity (changes of size and shape, often deformed after engulfing prey) and the difficulty in discerning surface striation. We conclude that the priority is for the species name Gymnodinium heterostriatum Kofoid & Swezy 1921, a new name that was proposed for Gymnodinium spirale var. obtusum sensu Dogiel 1906. The species Gyrodinium striatissimum (Hulburt 1957) Gert Hansen & Moestrup 2000 and Gymnodinium lucidum D. Ballantine in Parke & Dixon 1964 (=G. hyalinum M. Lebour 1925) are posterior synonyms. We propose Gyrodinium heterostriatum comb. nov. for Gymnodinium heterostriatum.